The atrial or ventricular pacing threshold is the minimum pulse energy (usually expressed as a voltage of a fixed width pulse) required to stimulate the muscle cells of the atria or ventricles to depolarize, i.e., to contract.
Pacing thresholds must be determined when the pacemaker, or other therapeutic pulse generator, is first implanted in the patient, and during subsequent follow-up examinations, to ensure that reliable "capture" is obtained while expending minimum energy. This is important since the pacemaker is battery powered and has a limited life. A conventional battery, depending on mode of operation, lead impedance, pulse amplitude, pacing rate and pulse width, may have a longevity typically ranging from four to ten years. Pulse amplitude and pulse width (which translate into energy consumed) are important factors in battery life.
Both the atrial and ventricular pacing thresholds must be measured if a dual chamber device is implanted. If a ventricular single-chamber device is implanted, then only the ventricular pacing threshold is required. If an atrial single-chamber device is implanted, then only the atrial pacing threshold is required. After a pacing threshold is measured, an appropriate pacing energy is chosen and programmed for the implanted pulse generator. The pacing energy is conventionally chosen to be two or three times the measured threshold so as to allow a safety margin for reliable capture.
Thresholds are measured at the time of implant with a pacing system analyzer when direct electrical access to the leads is possible. After implant, when the leads are not accessible, another method must be used. Conventionally, the threshold test is done with the aid of a programmer, which communicates with the implanted pulse generator via a telemetric link, at the same time that the patient's surface electrocardiogram (ECG) is viewed. Conventionally, either the atrial or ventricular threshold test starts from the previously programmed pulse amplitude and pulse width. The test is performed by automatically and progressively decreasing the pacing pulse amplitude by a fixed percent (e.g., 6%) on each test pace. The percent of decrease varies with the impedance of the lead involved. The pacing rate during the threshold test is set at a rate just above the patient's intrinsic rate to ensure that the pacing pulses will capture the heart.
When an atrial threshold is being measured, the amplitude of each decreased amplitude pulse can be annotated on the surface ECG trace. The amplitude of the last pulse to capture the heart represents the "pacing threshold." All pulses are delivered at the last programmed pulse width and pacing polarity. Unless halted manually, the test continues until either the pulse amplitude fails to a minimum predetermined voltage, or a fixed number of decreased amplitude pulses have been delivered.
The operator visually decides from the surface ECG when a loss of capture occurs, and thereupon manually terminates the test. The programmer displays the amplitude of the next-to-last pacing pulse before the termination of the test. If the test was not terminated immediately after that pulse which lost capture, the displayed amplitude will not be the true pacing pulse threshold. Therefore, the pacing pulse threshold must be confirmed by the operator by visual examination of the surface ECG. If the ECG trace is on paper, that portion of the ECG where a loss of capture occurred can be examined. If the ECG provides a trace into only a limited window of storage, then that portion of interest in the ECG may or may not be available, and the test may have to be done again. Thus, the conventional test procedure may be very time consuming. Pacing pulse thresholds also may not be determined appropriately due to operator error and this may have safety consequences for the patient.
In U.S. Pat. No. 4,969,462, issued Nov. 13, 1990 to F. J. Callaghan et al., for "Pacemaker With Improved Automatic Output Regulation", there is disclosed a threshold search by an implantable pacemaker which determines the pacing threshold by sensing the evoked potentials which follow the pacing stimuli and automatically sets the values of pacing energy accordingly. The pacing pulse is delivered between the tip electrode located inside the heart and the case of the pacemaker which is located under the skin on the patient. Sensing for evoked potentials is performed between the ring electrode located in the heart and the case. But in many patients only a unipolar lead, one with a tip electrode but no ring electrode, is available, and therefore pacing and sensing must be done through the same tip and case electrodes. In such a case, measurement of the capture threshold may not be feasible, because the pacing pulse induces potentials in the immediate area of the heart which are very much greater than those resulting from a heartbeat. Until the charges resulting from the pacing pulse dissipate sufficiently, reliable sensing is impossible.
To permit sensing with the same electrodes which are used for pacing, a triphasic stimulation waveform has been described by Whigham et al. in U.S. Pat. No. 4,903,700, issued Feb. 27, 1990, for "Pacing Pulse Compensation". Here the first and third phases of the pacing pulse are of one polarity and the second phase is of the other polarity, so that the net charge to the heart muscle is zero. This allows the same electrode which conducted the pacing pulse to sense the evoked potential. Due to the different surface treatments of pacing electrodes, the procedures described by Nappholz et al. in U.S. Pat. No. 5,172,690, issued Dec. 22, 1992 for "Automatic Stimulus Artifact Reduction For Accurate Analysis of the Heart's Stimulated Response", are advantageously incorporated to optimally adjust the triphasic waveform to reduce the stimulus polarization artifact. However, optimal triphasic waveforms may still be difficult to obtain with electrodes which have very high polarization characteristics.
Moreover, when pacing threshold searches are done routinely by the pacemaker, as described by Callaghan et al. and Nappholz et al., they may unnecessarily consume energy and shorten the life of the battery of the pacemaker due to the energy required to run the threshold searches. This is especially true today with the availability of drug-eluting leads which provide low and stable pacing thresholds.